The present invention relates to a device for transmitting or emitting high-frequency waves.
Devices for emitting electromagnetic waves, such as planar antenna elements, which are excited using a slot aperture for producing oscillation and, therefore, emitting high-frequency waves, have become widespread in radio link technology, satellite communications technology, and radar technology. They are used preferably in the microwave range, since this allows small component sizes and, therefore, simple realizations at low cost.
A common planar antenna device is presented with reference to FIG. 6A, in which a slot coupling is excited via a microstrip line (MSL) 10. To this end, microstrip line 10 has an abrupt end 10′ and therefore forms an open-ended line. A slot 14 is located in a ground surface 12 separated by a substrate 11, perpendicular to microstrip line 10, at a distance d of approximately ¼ of the line wavelength from abrupt end 10′ of microstrip line 10. Passage, i.e., coupling, of the magnetic field, which is at a maximum at this point, takes place through said slot. This field, which is also provided with an electrical field component, excites a planar antenna element 16—also called a patch element—to produce sympathetic vibration and nearly complete emission of high-frequency energy with a main direction of propagation which is orthogonal to ground surface 12. FIG. 6B shows a top view of the cross-section of the device according to FIG. 6A.
The disadvantage of this arrangement is that microstrip line substrates 11 become very thin at higher frequencies, e.g., 254 μm in a short range radar application (SRR) at 24 GHz, and do not have adequate structural stability to be expanded upon. For this reason, these substrates 11 must be joined with a rigid carrier material 18, as shown in FIG. 7A. For reasons of cost, this carrier material 18 is not suitable for use in high-frequency applications. Carrier material 18 is placed above ground surface 12 with a permanent connection therewith, and a cost-intensive recess 19 must be created in carrier material 18 to ensure that the antenna is capable of functioning in the region of coupling slot 14 or antenna element 16, so that antenna element 16 can be electromagnetically coupled via coupling slot 14.
To feed single antenna 16, a further conventional embodiment of a slot-coupled antenna uses a “buried” signal-carrying line 10 with an abrupt line end 10′ which is configured in the form of a “triplate line” and excites individual antenna 16 to produce emissions, also via a slot 14. Signal line 10 is located substantially plane-parallel between two ground surfaces 12, 13, whereby in the case shown in FIGS. 8A and 8B, microstrip line 10 is located closer to one of the two ground surfaces 12, 13, which results in an antenna arrangement with asymmetrical triplate feeding. In contrast, there are also arrangements with symmetrical feeding, i.e., embedded signal line 10 is equidistant between outer ground surfaces 12, 13. The symmetrical or asymmetrical triplate arrangement has the advantage that larger line elements can be hidden in a lower layer as buried structures, to reduce component size. When larger antennas are to be realized in particular which are composed of a large number of such individual antennas 16 in order to increase the directivity of the antenna, locating high-frequency line arrangements in layers located further downward make compact assemblies possible, since the feeding network of an antenna array takes up a significant portion of the required installation space.
Moreover, a buried feeding network does not negatively influence the emission characteristics of an arrangement of this type, in contrast to “open” distribution and feeding networks, in particular, which make a considerable contribution to parasitic emissions. Another advantage is the possibility of providing easily manufactured, multilayer arrangements, since their single layers have good high-frequency properties and carry the particular line structures to be buried. When suitable layer or substrate materials are used, such as ceramics, the connection with an additional mechanical carrier can be eliminated, since the multilayer arrangement has adequate structural stability. Low temperature co-fired ceramic (LTCC) substrates are particularly well-suited for use in this field.
The antenna arrangement described with reference to FIGS. 8A and 8B has the disadvantage, however, that the release of waves from an abrupt end 10′ of signal-carrying, center line 10 of the triplate structure is greatly enhanced. A considerable portion of the signal power can then disadvantageously propagate in substrate material 11, e.g., in the form of parallel plate modes or waveguide modes. If the multilayer arrangement is mounted laterally in a metallic carrier or housing, the excitation of waveguide modes is further enhanced. The propagation of waveguide modes is determined by their limiting frequency fg, the value of which depends directly on the distances from the bordering metallic walls.
The following relationship applies in general: Limiting frequency fg of a waveguide mode is shifted toward lower frequencies when the distance from electrically conductive, e.g., metallic, walls is increased. At the same time, the number of modes capable of propagating in a certain frequency band increases continually. If modes of this type are now excited in substrate 11 by open-ended line ends, the power emitted via antenna element 16 is reduced and couplings with other circuit parts within substrate 11, e.g., further antenna elements, are enhanced. This has a disadvantageous effect on the antenna characteristics and the overall system behavior.